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If you think Adam Sackler (played by Adam Driver) on HBO’s “Girls” is a curious character, just wait until you meet his sister.

Enter Gaby Hoffmann.

The former child actress, famous for movies like “Now and Then” and “Sleepless in Seattle,” left it all behind in 1999 when she enrolled at Bard College.

More than a decade later, she’s back and playing Caroline Sackler, the unstable, manic sister of Adam (Adam Driver), Hannah Horvath’s (Lena Dunham) live-in boyfriend.

“I don’t look at the role beforehand — Lena just said, ‘I’ve written this part of Adam’s sister with you in mind,’ and there wasn’t a decision to be made,” Hoffmann, 32, tells The Post. “I just said, ‘Of course I’ll come play with you.’ ” Hoffmann spent her childhood living in Manhattan’s Chelsea Hotel. She and Dunham hung out as kids, because Hoffmann’s stepmom, photographer Cindy Sherman, was friends with Dunham’s mom, artist Laurie Simmons.

The two reconnected a few years ago when Hoffmann ran into Dunham at a film festival at Brooklyn’s BAM theater — right before Dunham premiered her first film, “Tiny Furniture.”

The women stayed in touch and last year, Dunham called Hoffmann to tell her about the role of Caroline — the Sackler sibling who makes Adam seem subdued.

“We had a lot of serious, difficult stuff to do,” Hoffmann says about filming “Girls.” “But I just had so much fun. The most fun two days of my life were getting to fight with Adam Driver.”

In Sunday night’s episode, Caroline shows up on Hannah and Adam’s doorstep after a bad breakup, needing a place to stay. “I told you you’re never allowed to stay with me again after you tried to euthanize Grandma Helen!” Adam screams at his sis. “She asked me to!” Caroline shouts back. “She told me she was terminal!”

Like the character she plays on “Girls,” Hoffmann spent much of her twenties “lost,” she says.

“When I went to college in 1999, I left my agents and said goodbye to [acting],” she says. “Then I spent my twenties letting myself not decide and letting myself be lost. It was a very confusing time, and I did not enjoy it.”

Hoffmann lived in Italy for three months, interning for a chef and experimented with being a doula (a midwife of sorts) — her “Girls” character also worked as a doula — before realizing she missed acting. “So I said right then and there, OK I’m going to spend one year furiously acting, like taking whatever jobs,” she says. “[Before that], I would be broke and not audition for something because even the audition would provoke a whole spiral of uncertainty and anxiety about what I wanted. So I just thought: I’m not going to question it all, I’m going to say yes and dive in and have as much experience acting as I can — you know, barring total sh-t.”

Last year, Hoffmann moved to Fort Greene, Brooklyn, and took roles in films “All That I Am” and “Crystal Fairy and the Magical Cactus.” She had a guest spot in FX’s “Louie” and now has a four-episode arc on “Girls.” She recently shot a pilot of her own show.

“I’m really happy to be back at work,” she says. “I’m just relieved to have figured out that I am very interested in acting and love it. It’s great that I’m able to do it — I feel very lucky.”